Substantial effort has gone into developing and scientifically testing HIV prevention interventions, but these interventions do not affect the spread of the epidemic unless they are implemented effectively at community- based organizations (CBOs). Unfortunately, there has been little research examining how to successfully enhance the implementation of evidence-based interventions by CBOs. We spent many years developing and testing the efficacy of a community-level, HIV prevention intervention program for young gay/bisexual men, a group at high risk for HIV, called the Mpowerment Project. More recently we have been conducting a largely descriptive, qualitative prospective study of how 73 CBOs strive to implement the Mpowerment Project. We have also developed and have been providing a system of materials, training, and technical assistance (the Mpowerment Project Technology Exchange System, or MPTES) to help CBOs implement the intervention with fidelity to the original model. We have found that: (1) There is considerable variability, both between CBOs and within CBOs over time, in their implementation of the Mpowerment Project;(2) Issues at the organizational systems level play a substantially larger role in implementation effectiveness than we had anticipated;(3) Processes outside of the CBO (e.g., funder and community issues) often conflict with implementation of an innovative intervention;and (4) MPTES enhancements are required in order to be responsive to the ecology of barriers/facilitators to implementation that exist at multiple levels. Building on these findings, we have developed a conceptual model of implementation effectiveness that summarizes how complex systems, both internal and external to the organization, influence implementation effectiveness. We propose to conduct a prospective study of 80 CBOs that are striving to implement the Mpowerment Project, and, through quantitative methods, will test the model. Each CBO will be assessed at 4 time-points over two years, through online self-administered questionnaires and via telephone-administered surveys about the implementation of the intervention. We will also develop and implement an enhanced MPTES that addresses the potentially mutable variables in our conceputal model, and examine how utilization of the enhanced MPTES by CBO front-line staff, supervisors, Executive Directors, and funders improves their respective capacities to support effective implementation of the intervention.